The invention relates to a device for the transmission and/or reception of RF signals for magnetic resonance imaging (referred hereinafter in general as an “RF coil system”) which is constructed as an RF coil which is permanently mounted in a magnetic resonance imaging apparatus (body coil) or as a so-called dedicated RF coil (that is, as a separate RF coil which is to be arranged on or around a region to be examined, for example, a head coil, a shoulder coil, a flexible surface coil etc.), as well as to a magnetic resonance imaging apparatus provided with such an RF coil system.
Magnetic resonance (MR) imaging apparatus is used notably for the examination and treatment of patients. The nuclear spins of the tissue to be examined, aligned by a steady main magnetic field (B0 field) are then excited by a pulse-like B1 field which is orthogonal to the main magnetic field and has the MR or Larmor frequency. Moreover, for localization the nuclear spins are also subjected to gradient magnetic fields. The RF relaxation signals induced by the excitation are received and evaluated in order to reconstruct an image of the relevant tissue therefrom in known manner.
Essentially two types of construction can be distinguished: on the one hand there are the so-called axial systems in which the patient is introduced into an essentially horizontally oriented tubular examination zone. The magnetic fields are generated by magnet coils which are arranged along the circumference of the examination zone, the main magnetic field then traversing the patient in the direction of the longitudinal axis of the patient.
On the other hand, there are the so-called open MR imaging apparatus (vertical systems) in which the main magnetic field is generated, generally speaking, between two pole plates which are arranged one above the other and wherebetween a vertical cylindrical examination zone for a patient is defined. The main magnetic field (B0 field) traverses the patient essentially in a direction perpendicular to the longitudinal axis of the patient (that is, vertically). The patient then remains suitably accessible from almost all sides, that is, even during the imaging, so that interventional examinations can also be performed.
RF coil systems are permanently mounted in said systems (so-called RF body coils) in order to generate the B1 field (RF field) and to receive the RF relaxation signals; the configuration and positioning of such coils has a decisive effect on the image quality, notably the signal-to-noise ratio and the resolution.
Moreover, dedicated (separate) RF coils such as, for example, head coils, shoulder coils etc., are also used; these coils are also known as at least partly flexible surface coils or pads and can be arranged around or on the region of a patient to be examined.
In this respect it is very important that the overall examination zone to be imaged is traversed by an as homogeneous as possible RF field or that the reception characteristic of the RF coil system is as constant as possible in this zone. Furthermore, the field of view (FOV) of the RF coil system should extend as accurately as possible across the space of the at least substantially constant B0 field and the useful space of the gradient coils, generating the gradient magnetic fields, so that the resultant, defined useful examination zone (and only this zone) is exposed as completely and constantly as possible so as to be used for disturbance-free imaging.
For example, U.S. Pat. No. 6,150,816 discloses an RF coil system which includes at least a first, a second and a third RF coil which are electrically insulated from one another, can be separately activated and are arranged so as to overlap one another in the axial direction in such a manner that no magnetic coupling exists therebetween. The aim is to provide not only an improved signal-to-noise ratio but also an expanded and switchable field of view of the RF coil system.
However, this system has the drawback that the length of this coil system in the axial direction is comparatively large in comparison with the length of the field of view in this direction, that is, in particular when an as constant as possible variation and a steep drop of the RF field or the reception characteristic are desired at the axial ends of the field of view.